Ampeg svt 3

Jgrabs07 · #1 03-12-2013, 06:12 AM

Does anyone know if there is a way to actually tell if the mosfets need to be biased or is this a experiance kinda thing, will it hurt if i play one gig before i take it to the tech, so i got the bread to pay the man, i only need to be at stage volume as we have pa support for this gig, souldnt need more than 12 on the master, or is this not recomended, i have a pratcice rumble 350, but it doesnt even come close to the lows out of the 3 and the cabs hope the sound man knows his stuff. Thx in advance for the help

hodgy · #2 03-12-2013, 09:04 AM

How does it sound?
Checking the bias is actually pretty easy on these, all you need is a multimeter that will read millivolts. Or you can take it to a tech and have him take you for $60 for 5 minutes worth of 'work'. Man, I'm in the wrong business.

Jgrabs07 · #3 03-12-2013, 10:10 AM

Where do you have to put the leads to check the Mv, ive got a multimeter that reads Mv,

Jgrabs07 · #4 03-12-2013, 10:11 AM

I did some old school electronics school anyone remember rets electronics school

hodgy · #5 03-12-2013, 11:20 AM

Pull off the top and measure across the big white resistors in the middle. I usually take alligator clips and connect my black probe to a resistor leg in the middle so I have a hand free to move my other probe from resistor to resistor. An average of 25mV should be read across the 8 resistors (according to the old schematic). Don't blow anything up.

Jgrabs07 · #6 03-12-2013, 11:44 AM

Kool bro i can handle that thanks

Jgrabs07 · #7 03-12-2013, 11:59 AM

Oh,yea the sound is staticy across the whole volume range, or muted, thought it was tubes done all that, cleaned pots and tube saddles all good there, its an 06 model and still has the factory tubes, except for the au it was a groove tube, it tested good so i put it back in, my only other thought,was the groove tube mite be throwing the bias out, i think its a little hotter than the factory au

B-string · #8 03-12-2013, 12:23 PM

Keep in mind the only 100% reliable "tube tester" is the circuit it is used in. That is why these old service boxes loaded with tubes have "used" tubes in the boxes. If the tube "tested" good but the circuit didn't work properly a sub would be done. If the circuit then performed well the "old tube" may perform 100% in a different circuit.

JimmyM · #9 03-12-2013, 03:31 PM

Hodgy, my tech scared me off biasing my 3 Pro myself because he said the trimpot is very touchy and you can burn your amp up in short order. Truth or fiction?

dmrogers · #10 03-12-2013, 03:59 PM

Quote:

Originally Posted by JimmyM

Hodgy, my tech scared me off biasing my 3 Pro myself because he said the trimpot is very touchy and you can burn your amp up in short order. Truth or fiction?

This IS TRUE Jimmy. It is do-able but they are very sensitive.

So:

Measure all of them first.
Go slow.
Adjust in very small increments. I'm talking a hairs width.

This should go without saying but, make sure you observe all electronic safety rules. If you don't know them, then you should probably get someone else to do the biasing.

hodgy · #11 03-12-2013, 04:16 PM

Quote:

Originally Posted by JimmyM

Hodgy, my tech scared me off biasing my 3 Pro myself because he said the trimpot is very touchy and you can burn your amp up in short order. Truth or fiction?

True, but like any good tech he is trying to get your money. If you know it's touchy going in, then that's half the battle. Get a feel for the pot by going down first before bringing it up to operating voltage. Use alligator clips to leave a hand free to reach for the fire extinguisher.

JimmyM · #12 03-12-2013, 04:43 PM

LOL! Thanks guys.

hodgy · #13 03-12-2013, 04:46 PM

On second thought, just removing the screws will pretty much blow up everything. Better send it to me so I can charge $90/hr.

JimmyM · #14 03-12-2013, 04:59 PM

My local guy charges $60. And he's authorized.

hodgy · #15 03-12-2013, 05:00 PM

Quote:

Originally Posted by JimmyM

My local guy charges $60. And he's authorized.

But I'm double authorized.

JimmyM · #16 03-12-2013, 05:11 PM

Then you should charge $120.

Jgrabs07 · #17 03-12-2013, 05:50 PM

Thanks guys i think ill bite the bullet and take it to the tech, he said somewhere between 50-100 depending on what he finds, so im counting on the latter of coarse, well i really need have a gig this sat, four hour show, dont want to take chances with four 45 min sets, and its a benifet for a local ronald mcdonald house, i could plug into the board if something happens but id rather not risk a more exspensive repair later, thanks again guys